The Norse God of mischief. Also the villain in the movie "The Avengers" played by Tom Hiddleson that has completely taken over Tumblr as a whole and has turned previous fangirls of other media into Loki-serving mindless slaves.

Also another word for "troll" due to said event.

"You just got Loki'd."
"He's just Loki'ing, ignore him."
"That guy is such a Loki."

Loki is the Norse God of trickery, mischief, and change; a major character in Norse Mythology. Blood-brother of Odin and traveling companion and friend of Thor; his wife’s name is Sigyn, his children’s names are Hel, Fenris, Jormungandr, Vali, Narfi and Sleipnir. Loki is pleasing and handsome in appearance according to the Eddas. He is renown for his cleverness and for helping the Gods out after he has gotten them into trouble as well as his great ability to make them laugh and cheer them up.

He has domain over fire, and is a master magician, conjurer as well as shape-shifter (In some cases changing gender). Loki is bisexual, having sexual relations with both male and female in mythology. Bearing in mind Loki’s dark side as well as his good one, he is most valuable as a witty, entertaining friend, and a God to call upon.

Loki is not an evil malevolent being or the Norse version of Satan or some fallen angel. The later influence of Christianity on Norse culture resulted in changing the mythology portraying Loki as evil incarnate. This is not uncommon, Christianity often demonizes other religions gods.

Loki represents both our divine intelligence and also free will whereby we can choose for good or ill, and if we make a mistake to correct it. He typifies the human mind: on one hand clever, foolish, immature; on the other hand, he personifies the elevating, aspiring traits in human intelligence.

Loki is still worshiped today and one who worships or is dedicated to Loki as their primary God is called a Lokean. Compare to Odinist.

In pop culture:

Loki is a supervillian in Marvel comics.
Loki is a hero in Valhalla comic books.
Loki is a character on Mario and Luigi video games.
Loki is a character in the movie Domga
Loki is the spirit who inhabits the mask in the movie, “The Mask”.
Loki is a main character in the novel “American Gods”.
In Star Gate SG-1 Loki is a renegade Asgard who is responsible of many alien abductions in which he genetically experiments with human DNA in order to try and solve his races genetic degradation that is caused by excessive cloning.

Loki is the Norse Trickster god (originally a Jotun till he became Odin Allfather's blood-brother).
He plays some part in Ragnarok, the end of the world in Viking myth. For this reason he is often interpreted as a Satan figure.
Also, the only reason anyone knows any Norse myths is because of the Eddas, two writings on them. One was by Snorri Sturulson, and that writing is deeply influenced by Christianity. In Pagan beliefs, the end of the world is often seen as one being followed by renewal; it was seen differently by the pagans, and so Christian interpretation is, alas, inherently wrong.
Most Wiccans and other modern-day Pagans choose not to pray to Loki. He is the god of change, often change that is very necessary, hard to get through, and unwelcome at the time. Usually Loki's lessons are full of mischief, but the overall meaning is good-natured and helpful.
Loki is also, from a different perspective, a distinctly fey god. While the other Norse gods were all unchanging and fixed, as pagan gods usually are, Loki is the exact opposite, and it is well known that he did not come from Asgard as the Aesir did. Faeries, or fairies, are both destructive and constructive, being nature spirits, and Loki is very much the same. He both can and cannot be trusted. It depends very much on how anyone who prays to him interprets him, for he will appeal to that nature. "Watch what you wish for" applies to him in this case, for you will get what you want and find later that you shouldn't have wanted it.
Loki should not be confused with Satan; Satan represents destruction without cease. While Loki too is associated with fire, Loki's representation of it is the sort of fire that ravages a landscape and then allows many plants to grow there; the sort of destruction that makes way for a new beginning. The true Norse Satan figure is most likely Surtr, whose flaming sword, when drawn, signifies the end of the world. He comes from a land of eternal flame.
Loki is also an in-between god--not homosexual or transexual, but a shape-changer, who has spent time in the form of both a man and a woman, and has borne children. If he were pangender and/or pansexual, no one would be surprised. Androgyny and the telling of truth through lies (and jokes and parodies) are his domain. However, he dislikes being untrue to yourself to fit in.
He is traditionally supposed to have auburn hair. In the Tudor Humphries illustrations in Michael Harrison's 'The Doom of the Gods', he is shown in motley jester pants, harkening to his Trickster nature.
He is known as the "Father of Lies", among other things, but is not malicious.
Loki's first wife's name was Angrboda, and she bore three children: Fenrir, the oldest, was a giant wolf who killed Odin (blood brother to Loki and ruler of all the gods) in Ragnarok; the youngest was Jormungand, the Midgard serpent was a serpent who circled the world on the bottom of the ocean, devouring his own tail (an oroboros); he destroyed Thor (the Norse Zeus and Loki's constant companion). The middle child's name was Hel, which means death. Her lower body was that of a corpse, though her upper half was living. She rules Niflheim, or Helheim. Loki's second wife's name was Sigyn, and not much about her is known. They had two children, Vali and Narfi. When Loki was finally punished (in a manner similar to the crucifixion of Jesus), Vali was turned into a wolf and allowed to tear out Narfi's entrails, which were used to bind Loki to three stones. Skadi put a snake up above him and allowed the poison to drip onto his face. Sigyn held a bowl beneath it, to catch it; but whenever it became full, she had to turn away and empty it, and then the poison fell into his face.
Loki is more or less a nature god, not one of destruction; he is associated with the seasons and other natural changes, and like Mother Nature is unpredictable, playful, and sometimes dangerous. When prayed to it must be made clear afterwards that he is dismissed, otherwise the pagan has only himself to blame when he cannot find two matching socks the next morning.
His holiday is, fittingly, April Fool's Day.

Back before it came out in print again, I wanted a copy of Susan Kay's 'Phantom'. Since Erik wears a mask, and Loki's all about masks, I prayed to him to convince my dad to shell out 55 dollars on a used copy. Shortly afterward he ordered it, we walked into a bookstore, and I thought, "It would be really funny if I found that book here."
Unfortunately, we did, and bought it--for 50 cents! When a month later I heard it was coming back into print by popular demand, I knew Loki was dancing up in the clouds, laughing hysterically and my mortal stupidity. If I'd been patient, I would have got in good time, but Loki believes in everyone getting their wishes.
(True story.)

A sexy mofo with a tragic backstory, Loki was (at least in the Marvel cinematic universe, I do not vouch for comics) left to die by his birth father Laufey, the king of the Frost Giants of Jotunheim, because he was the runt. Odin took him in both out of compassion and because he thought adopting a Frost Giant would be useful to him in improving relations between their two kinds. Loki grew up being treated like something of an outcast by his adopted dad, who never told Loki that he was adopted and always favored Loki's brother Thor. This made Loki extremely jealous and desperate for his father's approval, which led to his becoming a villain. That and the fact he's the God of mischief and lies... so with that as your official job title, you probably aren't going to be the world's (realm's?) nicest person.

So me and other obsessive fangirls like myself are in love with him because he's beautiful and tragic and possibly even a good person under all that. And he's played by Tom Hiddleston who is God in a human form. You can't top that.

Loki is more popular than Thor because Thor is an arrogant-ish little goody two shoes and Loki is a MAN with fabulous dark hair and a sexy smile. He's also a bad boy, which makes him twice as hot. And British... so three times as hot.

Girl 1: TUMBLRRRRRRRRRRR.
Girl 2: Has she been Loki'd?
Girl 3: Of course she has. Loki is beautiful and perfect.
Girl 1: *cries* I DON'T HAVE A PROBLEM OKAY?

There are a lot of untrue statements here, so I might as well clear things up a bit.
Loki is a major character of Norse Mythology. He is often referred to as the trickster god, but he's technically a giant, son of the giant Farbauti. And despite previous definitions, he is NOT related to fire, that's a common misconception because the name Loki is similar to another name meaning fire.
Through the giantess Angrboda, He is the father of the half-dead woman Hel, the wolf Fenrir, and the world serpent Jormungand. He also once shapeshifted into a mare and gave birth to the eight-legged Sleipnir, which became Odin's favorite horse.
Loki is described as being handsome in appearance, but clever and deceitful. What makes Loki a truly fascinating character are his dynamic and unpredictable actions. Without the exciting, unstable, flawed Loki, the Norse Myths wouldn't have nearly as many interesting tales as they do today.
Loki often causes trouble for the gods. However, what's interesting is that it's often also Loki's cunning that gets them out of these messes. Many times, probably for the sake of fun, Loki helps out the gods while they are in tough situations. But at the end of the world, Ragnarok, Loki's true allegiance is tested, and he betrays the gods by fighting against them alongside the giants. At Ragnarok, he encounters the god Heimdall, and the two kill eachother.